In an apparatus such as a critical dimension SEM for measuring pattern size of semiconductor device with higher accuracy, experience in use of ultra-fine device pattern in recent years requires attaining of resolution of about 3 to 1 mm using the electron beam in an accelerating voltage of 1 kV or lower in view of protecting a specimen. In order to satisfy the requirement explained above, a diameter of the electron beam (spot size) on the surface of specimen must be reduced to a size of the required solution or less.
As a method of realizing above requirement, attention is focused to an aberration reducing method using an aberration corrector. In the charged particle optical apparatus represented by an electron microscope such as an SEM and a transmission electron microscope (TEM), a lens utilizing an electric field or a magnetic field is surely used in order to focus the charged particle beam. A rotary symmetrical lens which is generally used as an electric field lens or a magnetic field lens inevitably generates an aperture aberration such as chromatic aberration and spherical aberration. Accordingly, even when the spot size of charged particle beam is narrowed by increasing resolution of lens, a high quality image cannot be attained under the condition that aberration of charged particle beam is high. Therefore, substantial limit of resolution of the charged particle beam is decided at present depending on aberration.
As a means for removing aberration of the charged particle beam, an aberration corrector has been proposed. An aberration corrector is usually constituted with plural multipole lenses arranged in multiple stages and removes aberration included in the charged particle beam passing the inside thereof by generating an electric field or a magnetic field within the multiple lenses.
A non-patent document 1 (Nuclear Instruments and Methods in Physics Research, A363 (1995), pp. 316 to 325) discloses an aberration corrector for SEM based on a system utilizing 12-pole lens of four stages. Moreover, a patent document 1 (Japanese Unexamined Patent Publication No. 2004-265864) discloses an aberration corrector wherein octupole lenses, decapole to dodecapole lenses are allocated in multiple stages. The non-patent document 1 and the patent document 1 explained above describe the aberration corrector in the structure that an electrostatic multipole lens is used in the first and fourth stages, while an electrostatic•electromagnetic type complex type multipole lens is used in the second and third stages. In these documents, the electron beam which has been freed from aberration with methods explained below has been obtained.
(1) The electrostatic multipole lens in the first stage isolates the incident electron beam to the predetermined reference trajectories (x and y trajectories).
(2) A complex type multipole lenses in the second and third stages respectively removes aberration of electron beams isolated into the x trajectory and y trajectory.
(3) An electrostatic multipole lens in the fourth stage combines the electron beams isolated into the x trajectory and y trajectory into one electron beam.
In these aberration correctors represented by the non-patent document 1, formation of trajectories and correction of aberration are mainly executed using the electrostatic multipole lens and the electromagnetic lens is used as a complementary means. The reason is that the electrostatic lens assures, in comparison with the electromagnetic lens, higher reproducibility, for example, in focusing of image under the same conditions and easier control even when the setting values are changed and reset again to the original values after axis and intensity of the multipole field is once adjusted.